This invention relates generally to methods and apparatus for the elimination of free air or other gas from liquid flowing under pressure through piping. It is particularly applicable where the liqud flow is being metered.
In various industries handling flow of liquids under pressure, free air or other gas finds its way into the flow stream. An example is in the unloading of petroleum tankers by the use of shipboard pumps which discharge the liquid under pressure through piping to a shore installation. Due to operating procedures at the time of start-up of the pump and during stripping, large volumes of free air may be introduced into the piping. When it is desired to measure the product, as by the use of meters of the positive-displacement or turbine types, it will be evident that the presence of free air in the discharging liquid makes impossible accurate measure of the liquid product. In addition it may cause injury to the metering apparatus.
One method which has been used to eliminate free air from ship off-loading systems is to deliver the flow into relatively large shore tanks where it is permitted to stand under quiescent conditions at atmospheric pressure for a period such as from 2 to 4 hours, to permit dissipation of free air. The liquid in the tank may then be gauged to obtain a measurement of the quantity. This has the disadvantage in that it involves measurement in batch quantities, and in addition it is well-known that the gauging of large tanks involves inaccuracies. In addition, such large shore tanks are relatively expensive.
Another method which has been used is to employ relatively large retention or relaxation tanks into which the liquid is charged from the shipboard pump, and from which the liquid is pumped through a measuring meter. This method requires the use of a second pump to deliver the liquid from the retention tank to the meter, and again it involves the expense of a relatively large retention tank.